This invention relates to techniques and apparatus for color image sensing and in particular to improve color balancing.
Perceived colors depend almost exclusively on surface reflectance, yet the light that reaches the eye is a function of both surface reflectance and scene illumination. White balance actions are used in digital imaging systems to mimic a person""s ability to remove the effects of scene illumination from the perception of color.
An example of constant color perception in humans is that a white page looks white whether viewed under a yellow Tungsten light or under the blue sky. The human vision system somehow compensates for variations in illumination in order to achieve this result, although the processes through which this is attained are not well understood. A solid-state imaging system should implement a mechanism with a similar effect, otherwise the white page will look light blue under the latter condition and slightly yellow under the former. The goal of the white balance operation is to provide the imaging system with a compensation mechanism that allows it to reproduce constant white colors under different illumination hues.
In many imaging systems, much of the balancing process is performed on the analog sensor signal before it is recorded or transmitted. In particular, the white balance process is commonly described by the following two operations:
1. COLOR CHANNEL BALANCING. This operation compensates for the unequal spectral sensitivities of the different color channels in a digital image sensor. The color channels in a typical Bayer pattern color filter mosaic are: Red, Blue, Green-Red, and Green-Blue. The latter two are used to differentiate between green pixels adjacent to red pixels and green pixels adjacent to blue pixels.
2. COLOR CONSTANCY (also known as illuminant compensation). This operation attempts to mimic the human visual system""s phenomenon of keeping white objects constant. The illumination level in the scene is estimated and then the appropriate color transform is constructed. The estimation is generally carried out in software and fed back to the sensor.
White balance operations are usually performed in the analog processing chain of the sensor. They are difficult to implement and may contribute additional noise to the sensor data prior to digital conversion. Because the physical conditions that warrant white balance actions cannot be removed, the only recourse for reducing the hardware/software overhead that they entail, as well as minimizing their noise contributions, is to find alternative approaches.
Current image sensors are based on active pixel architectures and provide white balance actions as part of their post-capture analog processing chain. Color balancing operations require four input planes, one for each color channel, and this demands certain management overhead. The estimation of illuminant levels for color constancy operations is usually carried out in software and the linear transformation operation to produce the illuminant compensation is carried out in analog form which limits the dynamic range of the final compensated results.